The present invention relates in general to disk drive data storage devices. More particularly, the present invention relates to methods for improving the tracking capabilities of a servo system for positioning a read/write head with respect to a track on a disk drive data storage device.
In conventional compute data storage systems having a rotating storage medium, data is stored in a series of concentric or spiral tracks across the surface of a disk. Each track includes a number of data sectors. The storage medium can comprise, for example, a disk having a surface on which a magnetic material is deposited, such as conventional magnetic disks or magneto-optical disks. The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits are read from the disk surface by a head transducer suspended over the disk surface that can detect the variations in magnetic orientation as the disk rotates relative to the head and generate a fluctuating data signal.
Conventionally, a magnetic head is mounted on an arm or carriage that is incorporated in a servo system that moves the head, via an actuator, in a xe2x80x9cseekxe2x80x9d or xe2x80x9caccessxe2x80x9d function; i.e., the servo system moves the head to a selected track from a previous track. When the head reaches the desired track, the servo system commences a xe2x80x9ctrack followingxe2x80x9d function in which it accurately positions the head over the centerline of the selected track and maintains it in that position so that the head can write a series of data bits or, alternatively, read a series of bits from the track as the disk rotates under the head. Thus, the disk drive servo control system controls movement of the arm across the surface of the disk to move the head from track to track and, once over a selected track, to maintain the magnetic head within a given tolerance of distance over the centerline of the desired data track during read or write operations.
One such system is a sectored servo control system that is used to maintain the magnetic read/write head precisely over a desired track during a read or write operation. Such a servo system requires that servo information be prerecorded on the disk file. Servo information can be prerecorded on servo sectors located on each disk surface.
During both seeking and track following operations, the prerecorded servo information is sensed by the head and demodulated to generate a digital gray code and a position error signal (PES). The digital gray code includes track identification information and the PES indicates the position of the head away from the centerline of the track (i.e., an offset from the center of the track). The digital gray code and the PES are combined together to generate a measured position signal. The measured position signal is then used in a servo feedback loop to generate a control signal to move the head along a desired seek trajectory or towards the centerline of the target track.
In other words, each disk stores servo information in different sectors of the disk required for positional control. The embedded sectored servo method reproduces servo information written on the disk to determine the track number and the exact position of the head relative to the center of the track. A description of a general digital disk file servo control system is given by U.S. Pat. No. 4,679,103 granted to Michael I. Workman and titled xe2x80x9cDigital Servo Control System For a Data Recording Disk Filexe2x80x9d.
Embedded servo bursts are angularly spaced pie-piece-shaped sectors that are interspersed among the data sectors on the data disks. Alternatively, the servo signals may be recorded on a disk surface dedicated to servo signals. All of these mechanisms for servo control information are well known to those of ordinary skill in the art. The prerecorded servo information is normally written as servo sectors or bursts to the disks by a device commonly referred to as a xe2x80x9cservowriterxe2x80x9d or xe2x80x9ctrackwriterxe2x80x9d in a factory environment, before the disks are shipped to users. The servowriter is a machine dedicated to writing servo signals onto the disk""s surface. After the servowriter has recorded the servo information in the servo sectors, the disk is checked for quality by verifying the accuracy of the servo information. All of these mechanisms for servo control information are well known to those of ordinary skill in the art.
When a disk is used in a disk drive, the prerecorded servo information, including servo bursts, is read from the disks, demodulated, and processed by the servo control system. The results are then applied to the input of the servo electronics which in turn controls the current to an actuator such as a voice coil motor (VCM) and thus controls the position of the head over the disk surface.
Conventionally, A/B servo bursts are recorded at regularly spaced angular intervals around the disk surface. The servo burst fields are written symmetrically offset from and on respective sides of the data track center line by at least one-half of the track width. That is, the servo bursts do not overlap the track center line. Consequently, the difference between the relative voltage amplitudes, (VA-VB), as read by the head while track following, may be utilized as a direct indication of the distance and direction of the head from the track center line. In other words, when the drive head passes over these bursts, the readback amplitude of these bursts will be proportional to the portion of the width of the head that physically passes over the burst. For a head exactly on-track, both A and B bursts will read back with identical amplitude. If the head is biased toward the outside of the track, burst B will have higher amplitude than burst A, for example. If the head is biased toward the inside of the track, burst A will have higher amplitude, for example. By detecting the amplitude of A and B individually, and by calculating the difference between the amplitudes, the off-track PES signal can be generated.
Some disk drives accept removable disk cartridges that contain a magnetic storage medium upon which information can be written and read. The disk-shaped storage medium is mounted on a hub and both rotate freely as a unit within the cartridge. In order to rotate the disk, a spindle motor within the drive engages the disk hub when the cartridge is inserted into the drive. The spindle motor does this by first moving from an unloaded position to a loaded position when a disk cartridge is inserted into the disk drive. In the loaded position, the spindle of the disk drive motor contacts the hub of the disk cartridge. The spindle can then be rotated in order to rotate the hub and the storage medium of the disk cartridge. A shutter on the front edge of the cartridge is moved to the side during insertion into the drive, thereby exposing an opening through which the read/write heads of the drive move to access the recording surfaces of the rotating storage medium.
There are several causes for the position of a read/write head to be in error, or off-track, during a track following operation. One of the major components of head position error is called repeatable runout (RRO). RRO that occurs at the disk rotating frequency may be called 1F runout. There are several possible causes for 1F runout, such as an unbalanced spindle, a non-ideal spindle bearing, or centering error of a removable disk. The Workman U.S. Pat. No. 4,679,103 does not specifically deal with this problem. The servo control system disclosed in the Workman U.S. Pat. No. 4,679,103 does not have sufficient gain at the runout frequency to correct for the RRO error.
Particularly where a disk is removable from the drive, there is the possibility that the center of the circular tracks is not coincident with the center of the drive motor spindle, due to mechanical clearances causing a centering error. This causes a runout error or disturbance that is periodic at the frequency of the rotation of the disk, or 1F RRO. RRO can occur at various frequencies, such as 1F RRO or 2F RRO. It is typically desirable to reduce the runout.
In a feedback servo system that has finite loop gain at the 1F frequency, the sinusoidal 1F runout creates a sinusoidal off-track error that has an amplitude and phase relative to the amplitude and phase of the 1F runout. The 1F runout also has a phase with respect to the user data written on the disk""s data tracks. Thus, the off-track error also has a phase with respect to the user data written on the data tracks. The amplitude and phase of both the 1F runout and its resultant off-track error has traditionally been a random function of several mechanical factors, particularly the centering of the disk hub on the spindle motor of the disk drive.
2F runout is caused by anisotropic hygroscopic and thermal expansion of flexible disk media. It should be noted that the amplitude of 2F runout is proportional to track radius whereas the amplitude of 1F runout typically is independent of the track radius.
When the 2F runout is large enough with respect to the 1F runout, during track following, the actuator mechanics experience additional directional reversals over one revolution (e.g., four directional reversals instead of the usual two). These addition directional reversals increase the likelihood and severity of write-over encroachment and off-track error.
Friction in the actuator mechanics also contributes to off-track error, particularly when the directional reversals of the actuator take place. Thus, off-track error is, among other things, a function of 1F runout, 2F runout, and friction. The off-track error is most severe when the amplitude of the 2F runout and friction is large with respect to the 1F runout.
Although the art of read/write head positioning is well developed, there remain some problems inherent in this technology, particularly induced friction and offtrack error. Therefore, a need exists for a servo method and system that overcomes the drawbacks of the prior art.
The present invention is directed to a system and method for servowriting tracks having intentionally servowritten 1F runout on a data storage disk using a servowriter. A method according to one embodiment of the invention comprises misaligning the center of the spindle hub of a servowriter a predetermined distance with respect to the center of rotation of the spindle motor; mounting a data storage disk on the spindle hub; spinning the data storage disk up to an operational speed; loading the read/write head onto a surface of the data storage disk; and writing tracks on the disk surface via the heads, the center of the tracks being misaligned with the center of the spindle hub to form 1F runout having a 1F peak amplitude.
According to one aspect of the invention, the predetermined distance is a function of 1F runout of the spindle motor, tolerance on the diameter of the disk hub hole, and tolerance on the diameter of the spindle hub. According to another aspect of the invention, the predetermined distance is sufficient to dominate over centering errors in mounting the disk to the spindle hub. Preferably, the 1F peak amplitude is substantially equal to the predetermined distance. According to another aspect of the invention, the ratio of the 1F peak amplitude to a maximum expected 2F peak amplitude is at least about four.
According to further aspects of the invention, servo bursts are written on a first portion of each track at an angular spacing that is smaller than an average spacing for the track and servo bursts are written on a second portion of each track at an angular spacing that is larger than the average spacing. Preferably, the angular spacing changes in a substantially sinusoidal fashion around each track. According to another aspect of the invention, the servo burst having the largest angular spacing is disposed at the point of the track where the largest track radius occurs with respect to the center of the hub.
Another embodiment in accordance with the present invention includes a system for intentionally servowriting 1F runout on a data storage disk having a disk hub assembly, comprising a spindle motor; a spindle hub coupled to the spindle motor on which the disk hub assembly is mounted, the center of the spindle hub being misaligned a predetermined distance with respect to the center of rotation of the spindle motor; a read/write head for writing tracks on a surface of the data storage disk; an arm connected to the read/write head for moving the read/write head across the surface of the data storage disk so that the tracks are misaligned with the center of the spindle hub to form 1F runout having a 1F peak amplitude; an actuator connected to the arm for controlling movement of the arm; and a controller for controlling the actuator.
Another embodiment in accordance with the present invention includes a data storage disk for use in a disk drive, comprising a magnetic storage medium having concentric tracks written thereon, each track having a predetermined amount of intentionally written 1F runout having a 1F peak amplitude; and a hub attached to the magnetic storage medium, the hub having a substantially circular hole in its center, for engaging a spindle motor hub in the disk drive, so that the magnetic storage medium is rotated in the disk drive.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.